One of the major problems associated with electrical distribution cable is its tendency, over a period of time, to fail due to degradation of its insulation. The degradative processes involved in the failure of cables are associated with two "treeing" processes.
"Electrical treeing" is the product of numerous electrical discharges in the presence of strong electrical fields which eventually lead to the formation of voids within the insulation material. These voids resemble the trunk and branches of a tree in profile under microscopic observation, from which the descriptive terminology derives. As the trees formed by this process grow, they provide further routes along which corona discharges can occur, the cumulative effect being electrical breakdown of the insulation. Electrical treeing generally occurs when large voltages are imposed on the cable. The degradative results of the electrical treeing process can be precipitous such that the electrical cable can break down in a relatively short period of time.
The second type of treeing, known as "water treeing," is observed when the insulation material is simultaneously exposed to moisture and an electric field. This mechanism is much more gradual than electrical treeing, requiring an extended period of time to cause the degree of damage that affects the insulation characteristics of the distribution cable. However, since water treeing occurs at considerably lower electrical fields than required for the formation of electrical trees, this phenomenon is a leading cause of reduced service life of cables which allow water entry to the conductor region, whether through diffusion or some other mechanism.
It is known that water treeing can be reduced by the incorporation of an anti-tree additive (e.g., various organo silanes) in the insulation composition of the cable. Alternatively, this problem has been attacked by excluding water from the cable's interior by filling the interstices of the cable conductor with a dielectric material which effectively acts as a "water block." For the purposes herein, the term "interstices" includes the void space between individual conductor wires as well as voids between the wires and the insulation. The prior art teaches several compositions and methods for prolonging the service life of cables and reclamation of cables already damaged by the abovedescribed treeing phenomena.
U.S. Pat. No. 3,527,874 to Hayami teaches the use of silicon (sic) oil, or a silicon-hydrocarbon oil mixture, to fill the interstices between the conductor and insulation of an electrical distribution cable. Hayami teaches using low viscosity oils which can flow or exude through the cable's insulation layer. Thus the positive effects of the oil on the insulation would be lost when the oil leaks or exudes from the cable interior.
U.S. Pat. No. 4,372,988 to Bahder teaches a method for reclaiming electrical distribution cable which comprises: purging the cable with a desiccant gas; then supplying, in a continuous fashion, a tree retardant liquid, such a polydimethylsiloxane fluid, to the interior of the cable. Bahder does not, however, teach the addition of a curable organosilicone material to the interstices of the electrical distribution cable, and this disclosure also suffers from the above mentioned disadvantage in that the fluid can exude or leak from the cable. This reference addresses the potential loss of fluid by providing reservoirs which can maintain a constant fluid level, further adding to the complication of this system.
U.S. Pat. No. 3,939,882 to Gillemot teaches a cable reclamation apparatus which facilitates the injection of a reactive fluid mixture, such as polyurethane formulation, which cures to a paste like gel in the interstices of the cable. The initial mixture pumped into the cable reclaimed by this method has a viscosity on the order of about 200 cP at 70.degree. F. The polyurethane gel is formed by mixing two reactive components, thus requiring specialized equipment in order to deliver properly formulated uncured mixture to the cable interior. The formulations used in conjunction with this apparatus are, however, only useful for relatively short cable lengths due to the high viscosity and short cure times associated with the reactive fluid.
U.S. Pat. No. 4,008,197 to Brauer et al. teaches a method of forcing a low viscosity material into the internal free spaces of an insulated electrical device. This material acts to displace fluid contaminants and cures in situ to form a hydrophobic seal with good electrical properties.
U.S. Pat. No. 4,231,986 to Brauer et al. teaches a grease compatible material oil extended polyurethane composition which is useful in filling electrical devices. The compositions taught by Brauer are supplied to electrical devices as two component systems which require special metering devices to ensure that the proper formula is delivered to the cable. The grease compatible composition comprises: polyurethane precursor; mineral oil; and a coupling agent (or emulsifier) to stabilize the mixture.
U.S. Pat. No. 4,596,743 to Brauer et al. teaches a grease compatible extended polyurethane composition similar to the mineral oil extended polyurethane composition except that cyclic olefin extenders are used in the place of mineral oil.
The three patents to Brauer et al., cited supra, employ curing compositions which soon achieve a relatively high viscosity and must be pumped into the cable before cure has progressed to a significant extent. Otherwise, the attendant viscosity rise prevents efficient filling of all voids in the conductor region. The consequence of such an uncontrolled cure is that only relatively short cable lengths can be effectively filled using these compositions.
Various manufacturers currently provide cables which are filled with tar-like water block compounds (e.g., polyisobutylene) as a part of cable production. Typically, the water block compound is fed into the back end of a die through which wires are passed as they are twisted to form the conductor. Although such materials can work effectively to exclude water from the interior of these cables, the associated manufacturing process has proven quite difficult. This difficulty arises from the need for a precise balance between the water block feed rate and the conductor line speed. If too little compound is fed to the die, there will be voids in the water block where water can collect. If too much compound is fed, this will form a lump on the surface of the conductor and reduce the thickness of the insulation which is extruded over the conductor. This, in turn, would produce a weak spot in the insulation, making it more susceptible to electrical breakdown.